sendmail is a *nix only tool. Some emulators like cygwin may provide it as well, though I am not sure about that.. On windows your only option is to use anopesmtp instead of sendmail. It will pass the mail along to a configured smtp server, but keep in mind that unlike sendmail, it runs in anopes thread so if the server s slow to respond, hangs, or has to wait for a timeout, anope will be unresponsive for the entire duration anopesmtp is hanged up.